1. Field of the Invention
The present invention relates generally to communication over Ethernet-Class network and, more specifically to encoding methods for robust communication over noisy communications media.
2. Background
For transmission of digital signals some form of modulation is necessary to convert bit streams into an electrical waveform suitable for transmission over a communication channel. Modulation is used to minimize the effects of noise, match the frequency spectrum of the transmitted signal with channel characteristics and overcome some equipment limitations.
Modulation means to vary or change. To communicate a data signal over a communication channel, the data signal is impressed upon a constant wave called a carrier by varying or modulating this radio wave or carrier. Different modulation techniques, such as Amplitude Modulation (AM), Frequency Modulation (FM), Pulse Code Modulation (PCM) and so on, represent different ways to shape or form carrier waves.
Some simple and basic modulation techniques include Amplitude Shift Keying (ASK), Phase Shift Keying (PSK) and Frequency Shift Keying (FSK). Amplitude modulation represents the bits of information by changing a continuous carrier tone, where a continuous carrier is used to represent a zero, and a modulated signal is used to represent a one. In PSK modulation, the phase is shifted for the two different states. The major difference between ASK and PSK modulation schemes is that ASK is a linear modulation scheme, whereas PSK is a nonlinear scheme. FSK, on the other hand, consists of shifting the frequency of a sinusoidal carrier from a mark frequency to a space frequency. FSK is identical to modulating an FM carrier with a binary digital signal. FSK is the least susceptible to distortions during transmission. Because there are many different frequencies that could be chosen, there are many variations of FSK modulation, which can be defined by modulation indices.
Today, digital multimedia applications create an ever-increasing demand for broadband communication systems. Although the technical requirements for related products are quite high, the solutions must remain affordable to implement for consumer products. Orthogonal Frequency Division Multiplexing (OFDM) is a method that allows transmission of high data rates over extremely hostile channels at a comparable low complexity. OFDM has been chosen as the transmission method for the European radio (DAB) and TV (DVB-T) standard.
In OFDM, the subcarrier pulse that is used for transmission is rectangular. This has the advantage that the task of pulse forming and modulation can be performed by a simple Inverse Discrete Fourier Transform (IDFT), which can be implemented as an I Fast Fourier Transform (IFFT). Accordingly, in the receiver, an FFT can reverse this operation. According to the theorems of the Fourier Transform, the rectangular pulse shape will lead to a sin (x)/x type of spectrum of the subcarriers. The reason why the information transmitted over the carriers can still be separated is the so-called orthogonality relation giving the method its name. By using an IFFT for modulation, the spacing of the subcarriers is chosen in such a way that at the frequency where the received signal is evaluated, all other signals are zero. In order for the orthogonality to be preserved, the receiver and the transmitter must be perfectly synchronized. In other words, the receiver and the transmitter must both assume exactly the same modulation frequency and the same time-scale for transmission.
The OFDM symbols can be artificially prolonged by periodically repeating the tail of the symbol and precede the symbol with it. At the receiver this so-called guard interval is removed again. As long as the length of this interval is longer than the maximum channel delay all reflections of previous symbols are removed and the orthogonality is preserved. Accordingly, the OFDM transmission over a multipath channel can be achieved, since the transmitted symbols at a given time-slot and a given subcarrier are only disturbed by a factor which is the channel transfer function at the subcarrier frequency, and by additional white Gaussian noise.
However, despite of OFDM's provision of high data rates over extremely hostile channels, there still remains a strong need in the art to improve the existing frame structures for OFDM based signaling for data transmission over noisy communication channels, such as powerlines, and to enable, for example, a mix of TDMA and FDMA point-to-point and broadband access.